mbedTLS: extend days-to-keep

Extend days-to-keep to 60 for mbed-tls-pr-head and mbed-tls-pr-merge
jobs

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I4572d0186232b3429e959a6450014540cf6b6a2f
2 files changed